1. Field of the Invention
Embodiments of the present invention generally relate to sample adaptive offset (SAO) filtering in video coding.
2. Description of the Related Art
The Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T WP3/16 and ISO/IEC JTC 1/SC 29/WG 11 has developed the next-generation video coding standard referred to as High Efficiency Video Coding (HEVC). Similar to previous video coding standards such as H.264/AVC, HEVC is based on a hybrid coding scheme using block-based prediction and transform coding. First, the input signal is split into rectangular blocks that are predicted from the previously decoded data by either motion compensated (inter) prediction or intra prediction. The resulting prediction error is coded by applying block transforms based on an integer approximation of the discrete cosine transform, which is followed by quantization and coding of the transform coefficients.
In a coding scheme that uses block-based prediction, transform coding, and quantization, some characteristics of the compressed video data may differ from the original video data. For example, discontinuities referred to as blocking artifacts can occur in the reconstructed signal at block boundaries. Further, the intensity of the compressed video data may be shifted. Such intensity shift may also cause visual impairments or artifacts. To help reduce such artifacts in decompressed video, the HEVC standard defines two in-loop filters: a deblocking filter to reduce blocking artifacts and a sample adaptive offset filter (SAO) to reduce distortion caused by intensity shift. These filters may be applied sequentially, and, depending on the configuration, the SAO filter may be applied to the output of the deblocking filter. This in-loop filtering is one of most computationally intensive parts of the decoding process and may be approximately 15-20% of the overall decoding complexity.